The reason you're seeing this is because your connection profile probably contains instructions to verify server certificate, and OpenVPN3 (which is the default library for the community Android client) makes the assumption that if you use a certificate on the server side, then the client side also should have one. So it will ask you for one if there isn't one in the connection profile itself already.
However, if you know you are not going to use client certificates (but do keep using server certificate verification with for example remote-cert-tls server) you can add the following directive into your connection profile to let OpenVPN3 know that there is no client certificate expected:
setenv CLIENT_CERT 0
That should make it so the client doesn't ask and just connects. Of course, if the server is configured to require a client certificate, then it will fail now, but I'm going to assume that's not the case for your server.
